GEDmatch.com has a wonderful free tool called Are Your Parents Related which I have previously blogged about (click here). This function looks at your raw DNA results for long stretches where you have the exact same DNA on each side of a paired chromosome, known as Runs Of Homozygosity (ROH). In other words where you got the exact same DNA from each parent. I always check this for unknown parentage cases.
When you have ROH segments, it is expected that your parents are related. However there is one other way this can happen: in very rare cases, you can get a whole or partial chromosome from only one parent. This is known as uniparental disomy (UPD).
An example of UPD on chromosome 6 from the Are Your Parents Related (AYPR) tool.
How can you tell that this is the case? Likely it is UPD when you have only one ROH segment and it is for the whole chromosome like the image above or for one arm of a chromosome (from or up to the centromere). In the less than one hundred cases I have looked at, I have seen UPD only twice. Once a whole chromosome as shown above and once the long arm of chromosome 14.
UPD can result in some dangerous medical conditions as per Science Direct (click here). Please see a professional genetic councilor if you suspect you have this.
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UPDATE: 13-Dec-2020: FIXED! The code had not changed but the environment had, so an initialization was different. Thus my analysis that it was always the first few chromosomes helped the programmer solve the issue.
The “Are Your Parents Related Tool” (AYPR) has been an enormous aid to those adoptees who discover that they were a result of an in-family relationship. Thus it is very distressing to have gotten a report from a reader that the tool is suddenly showing less cM that are ROH than it used to.
My investigation has shown that the first few chromosomes have segments that are not marked as ROH when they should be and were in previous versions. The programmer who can fix this told me that this bug goes back perhaps as far as this past summer He does have a working copy from July, but is in the middle of a major new project. Thus he may not be able to attend to this until later this coming week,
In the meantime, here is an example of how this looks so you can try to make your own estimate. The results for a child of first cousins was shown in my blog post about this tool (click here for that post).
ROH for child of first cousins, buggy version on left, previous correct version on right (click for larger version)
To the left is how that looks now, while on the right is how it looked last year. Notice that the first three chromosomes on the left have not been included in the ROH listing. Also, the previous total was 215.3 which, when multiplied by 4, fit the first cousin scenario, now confirmed. The total without those first few chromosomes today is 126.8.
The cases from closer relations are even further off. A child of siblings had 744cM ROH last year but now gets only 465.6. A child from a father daughter pairing was was 750.4 and is now 547.9. In both cases the problem was the first four or five chromosomes were not having their ROH segments counted any more.
I will post an update on this blog post once the issue is fixed.
One of the first things I do when helping someone with their DNA results is to check if their parents are related. This can explain unusual patterns of matches, for example, all seemingly from one side.
GEDmatch.com has a nice tool called “Are Your Parents Related” (AYPR) in the”Analyze Your Data” blue panel (middle right of page) which looks for places in the specified kit where the DNA is identical on both chromosome pairs, maternal and paternal. This happens when you inherit the same segment of DNA from each parent because they are related. We call this a homozygous run which is a fancy way of saying a stretch of identical DNA on both sides.
CeCe Moore specializes in helping people who make this discovery. Click here for the informational brochure she helped Brianne Kirkpatrick, genetic counselor, create. It includes where to get emotional support.
My goal is to help you figure out what the DNA means yourself. Can you deduce what the relationship of those parents is? Well a very simple rule of thumb is to multiply the shared DNA from AYPR tool by four and look up that new total at the DNA painter calculator for the possibilities. Then do further family DNA testing to confirm.
Why does this work? Let’s look at the numbers. Suppose your parents share 25% of their DNA. They will pass about half of that to you, so 12.5%. However only about half of that will be the same DNA so it will show up as about 6.25% on the AYPR tool.
Look at the image. The total is 215.3 when you multiply by 4 you get 861.2. You might look that up before you read on …
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